I want to knit the big bad baby blanket from the SnB handbook, I have some Celestial Blue Faced Aran yarn, I guess it is worsted weight. The pattern calls for Koigu Designs merino (100% wool) knit with two strands. If I just knit the pattern using one strand of the yarn I have, would that work?

Just check your gauge with the single strand of what you have and see if it is comparable to the pattern gauge. Because it is a blanket, gauge isn't as crucial as it would be if it were a garment or something that needed to fit well, but as long as your gauge is about the same, you should end up with roughly the same size finished blanket and you'll be good to go.